Current glass tempering machines employ what are known as oscillating roller furnaces in which glass is heated mainly by radiation. In the tempering process the temperature of the glass is increased above the softening point of glass in order to enable the glass to be tempered. Said temperature is between 610 and 625.degree. C. depending on the thickness of the glass. The glass is then cooled at desired speed typically using forced convection whereby air jets are blown at the glass from above and from below. This method enables high heat-transfer coefficients, necessary when thin glass is concerned in order to achieve a sufficient temperature difference between the surface and center of the glass. Examples of oscillating roller furnaces are disclosed in FI patents 83,072 and 86,407.
A typical problem with tempering furnaces is that the furnace has to be operated manually as to the final temperature of the glass. Glass temperature is controlled afterwards based on the final result of the heating of a previous load, The temperature is typically measured by a pyrometer disposed outside the furnace between the furnace and a cooling unit, whereby the furnace has to be controlled by intermediate measurements such that the operator increases or decreases the heating time of the glass in the following load. Furthermore, attempts are made to keep the furnace in a constant state. The system operates when the furnace has reached a balanced state, the loads being of equal weight and size. The problem is quite serious if the size of the series to be heated in the furnace is small, whereby the load and/or temperature of the furnace has to be changed frequently or there are frequent pauses etc. A further problem is that the glass made in the furnace is very operator-dependent, i.e. if the operator is inexperienced or has insufficient basic schooling required for operating the furnace, the final result can be quite inferior.
A basic reason for the problem is that, because the glass is continuously being supported by the rollers, heat is transferred from the rollers to the glass by conduction which is predominant relative to other heat transfer, particularly at the initial heating stage when the temperature difference between the hot rollers and the glass is great. A further problem is that the average roller temperature does not stay constant between successive loads, but instead decreases load by load if the furnace is loaded with normal loads. When the average roller temperature falls, the heating time of glass has to be increased until the furnace reaches its so-called constant state. Thus, said change in heating time is usually carried out manually based on the temperature taken from the previous load or based on the quality of the previous load.
Furthermore, glass optics is at its best when the glass is as cold as possible throughout, but then again breakage and the tempering result is better the hotter the glass has been heated. Thus, in practice compromises have to be made between the optical quality of the glass and the breakage degree, and the operation has to be limited to a very narrow range. In this case it would be very important to maintain the temperature of successive loads as constant as possible.